Method of manufacturing a component with a protective arrangement which prevents aluminizing or chromizing during gas diffusion coating

ABSTRACT

A method for selectively protecting a component made of an iron-base alloy, cobalt-base alloy or nickel-base alloy with a protective arrangement which protects select areas of the component from aluminizing or chromizing during gas diffusion coating, wherein a first layer acts as an interlayer and a second layer acts as a getter layer for reaction gases. The method comprises the steps of depositing a first layer of slip cast material comprising oxide ceramic particles carried in a low-carbon, halide free, liquid vehicle; depositing a second layer comprising metal or a metallic slip, which comprises at least 50% by weight of the base metal of the component and all major alloy constituents of the component; aluminizing or chromizing by gas diffusion coating; and removing the protective arrangement from the component.

This is a Divisional of application Ser. No. 08/363,225, filed Dec. 23,1994 now U.S. Pat. No. 5,523,165.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to a component in an iron, cobalt or nickel-basealloy having a protective arrangement to prevent aluminizing orchromizing during gas diffusion coating that includes a first layerserving as an interlayer and a second layer serving as a getter layerfor reaction gases.

When producing protective coatings, such as aluminum or chrome diffusioncoatings, by means of the powder packing method in accordance with U.S.Pat. No. 3,079,276 or U.S. Pat. No. 3,801,353 or by means of the vaporphase deposition method in accordance with U.S. Pat. No. 4,132,816 orGerman Patent Document DE 38 05 370, it has proved difficult to protectcomponent sections from aluminizing or chromizing that should not beexposed to the coating atmosphere. In conventional protectivearrangements the first layer consists of a fill of Al₂ O₃ particles anda getter layer consisting of a metal-metal oxide powder blend. Suchpowder fills are commercially available.

At coating temperatures above 900° C. it is especially difficult toensure high absorption capacity of the conventional protectivearrangement for the elements to be deposited and for the aggressivehalide-containing reaction gases without contaminating, etching,corroding, diffusion hardening, carbon- or oxygen embrittling or in anyother manner altering the component area and surface to be protected.Also, at such elevated temperatures, components of the protectivearrangement may chemically react to alter the protected componentsurface, which again gives rise to problems. Additionally, particles ofthe first layer of the protective arrangement may bond with thecomponent by sintering or diffusion.

Conventional protective arrangements, such as shielding jeopardizedcomponent areas by powder fills or arranging jeopardized component areasin powder fills outside the reaction chamber have so far provedpracticable at negligible scrap rates only for gas diffusion coatingbelow 900° C. In conventional protective arrangements, scrap is producedas a result of creep coating induced by the highly active, greatlyreactive and scattering coating atmosphere at elevated temperatures.Also, damage to the protected component surfaces several micrometersdeep and varying with time and temperature has been noted.

The present invention aims to provide a generic protective arrangementthat overcomes the noted disadvantages and ensures complete protectionof the component also when gas diffusion coating at temperatures above900° C. The present invention also provides a simple low-cost method formanufacturing, depositing and removing the protective arrangement.

It is a particular object of the present invention to provide anarrangement in which the first layer is a slip cast layer composed ofoxide ceramic particles carried in a low-carbon vehicle free fromhalide, while the second layer is a metal layer or a metallic slip castlayer, where the metal layer exhibits at least 50% by weight of the basemetal fraction of the component and contains all major alloyconstituents of the component.

This protective arrangement provides an advantage in that it ensuresfull protection for the component area to be protected from aluminizingand chromizing during vapor-phase deposition also at processtemperatures above 900° C. Suitable matching of the material compositionof the gettering and sealing metal layer, which is entirely free ofoxide ceramic constituents, consists at least 50% by weight of thecomponent's base metal and contains all major alloy constituents of thecomponent, provides a further advantage in that this metal layerexhibits neither diffusion effects nor chemical reactions with thecomponent material to be protected in the surface zones of the maskingprotective arrangement, especially so when the metal layer is chemicallyidentical with the component material. The metal layer in the inventivecomposition protects the material of the component from aluminizing orchromizing by absorbing the elements (Al or Cr) and metal halidespresented in the vapor phase, while on account of its metalliccomposition it will not alter the surface zone of the protectedcomponent area. Another advantage is provided in that the componentsurface is not depleted of elements easily removed by diffusion, such asaluminum, titanium or chrome, considering that in accordance with thisinvention, these are presented in sufficient concentration by the metallayer.

In a preferred aspect of the present invention the protectivearrangement will not cover the component in its entirety, but onlyselectively, so that the surface areas of the component to be aluminizedor chromized are fully exposed to the reaction gases.

To still improve on the action of the protective arrangement, thearrangement preferably exhibits several layers reacting with the coatingatmosphere. These layers have metal particles of varying compositions,where the metal layer that in terms of composition corresponds most tothe component alloy is preferably arranged closest to the componentsurface. The metal layer which on account of its composition or porosityhas a pronounced gettering effect on the reaction gases is used as thecentral layer, while the outer layer is a maximally gastight layer,preferably metal foil or metal sheet masking.

In a further preferred aspect of the present invention the metal layeris arranged as a conformal metal foil or conformal metal sheet on theinterlayer. Conformation to contour is here achieved either by pressingthe metal foil onto the interlayer or by subsequently filling the spacebetween the metal sheet and the first layer with metal slip. This aspectof the present invention provides an advantage in that the metal layercan be given an extremely effective gas sealing effect relative to thereaction space.

An additional contour-hugging masking sheet can be used to seal themetal layer preferably relative to the reaction space. A masking sheetof this description adds to the soundness of the protective arrangementand simultaneously facilitates the manipulation of the components whenpreparing for and performing the gas diffusion process. The maskingsheet is here designed to split preferably along the molding base. Inthis manner the sheet is advantageously removable in halves from thecomponent following the gas diffusion coating.

In a particularly preferred aspect of the present invention the basemetal fraction in the metal layer does not deviate by more than 20% byweight from the base metal fraction of the component. This restrictionadvantageously ensures that in terms of material composition, the metallayer and the component are more precisely matched to one another andthat diffusion voids or such other lattice defects as may otherwisearise in the presence of differing work functions in the rim areasbetween the metal layer and the component are avoided.

The fraction of chrome content in the metal layer should deviate from apotential chrome fraction of the component by not more than 20% byweight to avoid defects in the component surface. Similarly, thefraction of a Mo, Co or W content in the metal layer should vary from apotential Mo, Co or W content of the component by not more than 10% byweight, while the fraction of an Al or Ti content in the metal layershould vary from a potential Al or Ti content of the component by notmore than 5% by weight.

For clean separation of the protective arrangement from the componentfollowing vapor-phase diffusion, the interlayer preferably consists ofAl₂ O₃. The thickness of this layer preferably runs between 0.3 mm and 2mm.

A method for manufacturing a component in an iron, cobalt or nickel-basealloy with a protective arrangement to prevent aluminizing or chromizingduring gas diffusion coating with a first layer as an interlayer and asecond layer as a getter layer for reaction gases uses the followingprocess steps:

a) Deposit a first slip cast layer composed of oxide ceramic particlescarried in a low-carbon vehicle free from halide;

b) Deposit a second layer of metal or metallic slip, where the basemetal of the metal layer contains at least 50% by weight of the basemetal fraction of the component and the metal layer exhibits all majoralloy constituents of the component.

This method provides an advantage in that it is suitable for bulkproduction, especially so as the slip casting process is a readilymanageable technique giving reproducible coating results andthicknesses. For this purpose, a slip casting layer of preferably oxideceramic particles is deposited by means of a vehicle consisting ofpolyvinyl alcohol or polyvinyl acetate and water. The metal layer as agettering layer can be deposited to hug the contour also by plasmaspraying, electroplating, painting with inorganic metal paints, vacuumvapor deposition or foil plating.

In a preferred aspect of the inventive process the component ispartially fitted into a closely contoured outer sheet which does notreact chemically with the component material and which in compositioncorresponds to the metal layer and then the space in between is fully orpartially filled with slip for the first layer, or when the first layerhas previously been deposited, with the material for the second layer,and the slip is then dried. This practice has shown that the slipbetween the closely contoured outer sheet and the component, ifdeposited simply as a seam like a weld bead, will provide perfectprotection. With this process variant a compact assembly of protectivecoating layers of the protective arrangement and a closely contouredouter sheet for masking can be economically manufactured. The slip castlayer can here consist of oxide ceramic particles (first layer orinterlayer) and be deposited preferably by means of a vehicle consistingof polyvinyl alcohol and water.

This protective arrangement is preferably used for protecting componentsor component areas from aluminizing or chromizing during gas diffusioncoating at temperatures above 900° C. and a high surface layer activitygreater than 20% by weight Al or Cr for the layer deposited by gasdiffusion. It has here been seen that practically perfect identity ofthe gettering metal layer with the component material alone willreproducibly ensure that the surface of the protected component arearemains free from damage.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures below show surfaces in the masking areas of variousprotective arrangements.

FIGS. 1 and 2 show ground and polished microsections of component areasprotected during vapor-phase aluminizing by conventional protectivearrangements, and

FIGS. 3 and 4 show ground and polished microsections of component areasprotected during vapor-phase aluminizing by inventive protectivearrangements.

DETAILED DESCRIPTION OF THE DRAWINGS EXAMPLE NO. 1

A jet engine turbine blade in RENE 80, a nickel-base alloy, is protectedin the blade root section, the fir-tree profile of which is not to bealuminized or chromized, by means of a conventional protectivearrangement consisting of an interlayer of oxide ceramic powder and agettering second layer consisting of a metal-metal oxide powder blendsuch as it can be procured from Alloy Surfaces, Wilmington, USA orTurcochrom, Israel. The protected arrangement is exposed to vapor-phasealuminizing at 1060° C. for a duration of 6 hours, with a high surfacealuminum content of 33% by weight aluminum being maintained.

The soundness of protection is illustrated in FIGS. 1 and 2, which at500× magnification show microsections taken at right angles with thecomponent surface of the protected areas. FIGS. 1 and 2 clearly showcrystallites of the base material 1 with segregations of readilydiffusible alloy constituents (a dark grey in the microsection) fromaluminum, chrome or titanium in the crystal volume. Near-surface areas 2appear bright in the microsection and no longer show segregations. Thereadily diffusible alloy constituents have disappeared from these areasas a result of the imperfect protective action of commercially availablemasking systems. The component surface is depleted of essential alloyconstituents.

EXAMPLE NO. 2

A jet engine turbine blade in RENE 80, a nickel-base alloy is maskedwith the inventive protective arrangement in the blade root area, thefir-tree profile of which is not to be aluminized or chromized. For thispurpose, a first layer of an Al₂ O₃ slip 0.8 mm thick is deposited, theslip consisting of Al₂ O₃ particles of a mean particle size under 50 μmin a vehicle consisting of polyvinyl alcohol and water. Thereafter, aslip casting layer 1.5 mm thick of metal powder is deposited, the metalpowder consisting at least 50% by weight of the base metal of thecomponent and containing all major alloy constituents. In this examplethe metal particles contain 15% Cr, 4% Al, 3.5% Ti, 4% Mo, 15% Co, allby weight, the remainder being Ni. The slip here contains a vehicleconsisting of polyvinyl alcohol and water. The protective arrangement isfinally masked with an outer metal sheet 0.5 mm thick in a Fe-basealloy. The protective arrangement is exposed to vapor-phase aluminizingat 1060° C. for a duration of 6 hours, where a high surface aluminumcontent of 33% by weight aluminum is maintained.

The effectiveness of protection is illustrated by FIGS. 3 and 4, whichshow microsections at 200× and 500× magnification, respectively, takenat right angles with the component surface of the protected areas.Crystallites 5, 6, 7 with sharply contrasted grain boundaries 3 areapparent in base material 1. No zone of removal by diffusion is obviousat the grain boundaries or in the near-surface area. No cracks areapparent in the surface even under 500× magnification (FIG. 4).

EXAMPLE NO. 3

A jet engine turbine blade in RENE 80, a nickel-base alloy is maskedwith the inventive protective arrangement in the blade root area, thefir-tree profile of which is to be excepted from aluminizing orchromizing. For this purpose, a first layer of an Al₂ O₃ slip layer 0.8mm thick is deposited, where the slip consists of Al₂ O₃ particles of amean particle size of under 50 μm in a vehicle of polyvinyl alcohol andwater.

Thereafter, a metal layer is deposited, consisting of a conformal powdermetallurgically manufactured molding 6 mm to 10 mm thick, where thespace between the PM molding and the first slip casting layer is filledwith a metal powder slip. The molding is sintered for a protectivearrangement of nickel-base alloy (RENE 80) component from a nickel-basemetal powder of a mean particle size under 50 μm, with the metal powderof the present example containing 15% Cr, 4% Al, 3.5% Ti, 17% Co, all byweight, the remainder being Ni. The protective arrangement is exposed tovapor-phase aluminizing at 1060° C. for a duration of 6 hours, with ahigh surface aluminum content of 33% by weight aluminum beingmaintained.

In terms of surface quality the effectiveness of protection provided byexample No. 3 corresponds to that of example No. 2. No surface defectsor damage are noted.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration andexample, and is not to be taken by way of limitation. The spirit andscope of the present invention are to be limited only by the terms ofthe appended claims.

What is claimed is:
 1. A method for selectively protecting a componentmade of an iron-base alloy, cobalt-base alloy or nickel-base alloy witha protective arrangement which protects select areas of the componentfrom aluminizing or chromizing during gas diffusion coating, wherein afirst layer acts as an interlayer and a second layer acts as a getterlayer for reaction gases, comprising the following steps:a) depositing afirst layer of slip cast material comprising oxide ceramic particlescarried in a low-carbon, halide free, liquid vehicle; b) depositing asecond layer comprising metal or a metallic slip, which comprises atleast 50% by weight of the base metal of the component and all majoralloy constituents of the component; c) aluminizing or chromizing by gasdiffusion coating; and d) removing the protective arrangement from thecomponent.
 2. The method of claim 1, wherein the component is partiallyfitted into a conformal outer metal sheet which does not chemicallyreact with the component, the composition of which corresponds to thatof the second layer, andwherein the space in between the component andthe outer sheet is subsequently partially or completely filled with theslip cast material for the first layer and the slip cast material isthen dried.
 3. The method of claim 1, wherein the vehicle comprisespolyvinyl alcohol and water or polyvinyl acetate and water.
 4. Themethod of claim 1, comprising gas diffusion coating the component attemperatures above 900° C., wherein the coating deposited by gasdiffusion has a surface layer activity greater than 20% by weight. 5.The method of claim 1, wherein subsequent to the step of depositing thefirst layer, the component is partially fitted into a conformal outermetal sheet which does not chemically react with the component, thecomposition of which corresponds to that of the second layer, andthespace in between the component and the outer sheet is partially orcompletely filled with the metallic slip for the second layer and themetallic slip is then dried.